Defects aid mother-of-pearl's assembly, according to new research

Scientists have finally uncovered the structural secrets of mother-of-pearl, or nacre, the organic-inorganic composite material that forms the smooth, shiny layer found inside the shells of many mollusk species.

According to a new study, published Monday in the journal Nature Physics, structural defects in the material's self-assembly process ensure nacre's incredible uniformity and strength.

Impressively, mother-of-pearl is the product of a disorganized synthesis process.

The process begins when individual cells begin depositing bits of material simultaneously at different locations. During the early stages of construction, the material is not at all uniform.

"In the very beginning, the layered mineral-organic tissue is full of structural faults that propagate through a number of layers like a helix," study co-author Igor Zlotnikov said in a news release.

"In fact, they look like a spiral staircase, having either right-handed or left-handed orientation," said Zlotnikov, a research group leader at Dresden University of Technology's Center for Molecular Bioengineering.

"The role of these defects in forming such a periodic tissue has never been established," Zlotnikov said. "On the other hand, the mature nacre is defect-free, with a regular, uniform structure. How could perfection emerge from such disorder?"

To find out, Zlotnikov and his colleagues worked with scientists at the European Synchrotron Radiation Facility in Grenoble, France, to image and compare the structures of both early and mature nacre.

Using the French facility's high-powered imaging technology, synchrotron-based holographic X-ray nano-tomography, researchers were able to observe the growth of nacre at extremely fine scales.

The state-of-the-art imaging technology also allowed researchers to analyze the material's growth in three dimensions.

"The combination of electron dense and highly periodical inorganic platelets with delicate and slender organic interfaces makes nacre a challenging structure to image," said co-author Alexandra Pacureanu, researcher with the X-ray Nanoprobe group at ESRF.

"Cryogenic imaging helped us to obtain the resolving power we needed," said Pacureanu, a researcher with the X-ray Nanoprobe group at ESRF.

To analyze the complex imaging data, the researchers used what's called a segmentation algorithm to train computer neural networks to distinguish different layers of the mother-of-pearl material.

The novel algorithm allowed researchers to identify the behavior of individual defects as the material matured. They found screw-shaped defects seemed to influence the behavior of other defects from surprisingly long distances.

For example, right-handed and left-handed defects migrated toward one another until they collided and cancelled each other out. The phenomenon allowed the material to become increasingly uniform as it matured.

Researchers said they suspect this so-called periodic formation process may explain the uniformity of other similarly strong biogenic structures.

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